The present invention refers to a method of sterilizing a container.
Within the food industry there is a great need to be able to sterilize containers which are manufactured as closed containers and which when subsequently filled can be opened, filled, and sealed under sterile conditions. Furthermore, there is a great interest from the market for an aseptic bottle of polyethylene terephtalate (PET). The aseptic treatment of a package material such as polyester is rather unusual since this type of material is mostly used for soft drinks and the like.
Electron guns have been used for food containers for the purpose of sterilizing relatively thin containers from the outside, and for several years the sterilization method has been contemplated for further use within the packaging industry. When a commercial plastic bottle is irradiated from one side with a relatively low energetic and thus cheap commercial electron generator it is relatively simple to sterilize the cubic content within the bottle as well as smooth surfaces on the inside of the container. However, when other places in the bottle, such as its neck and bottom part, are concerned the material at those places is so thick and the radiation due to the low energy is so low that a sufficient lethal effect cannot be achieved. Different systems have thus been developed in order to obtain an effective sterilization within this type of container, the sterilization being achieved via the open part of the container.
The sterilizing effect of electrons has been known for a long time. Electron irradiation is a generally known sterilization method, and the mechanism behind the lethal effect has been thoroughly studied. The main lethal mechanism of the irradiation is that these electrons within a cell break bonds in the DNA chain.
One problem with electron beam sterilization has constantly been that ozone always is formed during the irradiation of the material to be sterilized. When sterilizing by this method, which mainly is performed on non-closed containers in close connection with the filling of the container, great efforts have been spent on taking care of this poisonous gas. This can be achieved by passing the container through a heater with sterile air, the ozone formed being inactivated and/or ventilated away to the greatest possible extent. Alternatively, surplus ozone can be removed with nitrogen or the sterilizing process can be performed in vacuum. However, all these procedures are costly.
A further problem is that the ozone formed in turn can react with the package material, and the reaction products obtained can give an off flavor when solubilized from the material. Thus, the ozone generated is considered to result in product limitations in that sensitive products are more difficult to pack.
The purpose of the invention is to provide a method of the kind mentioned by way of introduction, which method allows a more effective sterilization of closed containers in a cheap and simple way, the problems mentioned above being eliminated.
In order to achieve this purpose the method according to the invention has been given the characterizing features of claim 1.
In order to further explain the invention reference is made to the accompanying drawing, in which
FIG. 1 shows the killing of microorganisms as a logarithmic reduction (LGR) after electron irradiation of sealed PET bottles containing air (empty rectangles) or helium (hatched rectangles).
During an electron irradiation in air the oxygen therein is converted to ozone. However, the different effects caused by ozone is not as well studied as those which have been caused by electron irradiation. Ozone is known to be a strong oxidant of organic substances, but the prospects of using ozone has been limited by the high investment and operating costs for its production. However, it is considered that the ozone moleculesxe2x80x94in the form of activated oxygenxe2x80x94by means of chain reaction give rise to what is called free radicals which result in that biomolecules (DNA, RNA, enzymic and structural proteins, and saturated fatty acids, etc.) are changed and destroyed. Thus, several, maybe all enzymes in a cell can be influenced by the oxidative change of their catalytic or allosteric centra.
The air which has been activated by means of electron irradiation is according to the invention utilized in that the container is sealed before the electron irradiation is started and the ozone formed is retained within the closed container for the purpose of sterilizing the same. Thus, an environment rich in ozone is produced, which during a suitable and necessary period of time is allowed to exert its effect so that a satisfactory sterilization is obtained.
However, it has been found that the ozone formed has half-lives which are very dependent on its environment, i.e. the material in the vicinity of the O3-molecules. The half-life depends on such parameters as the humidity of the air as well as the temperature and it can vary from about ten seconds to several days. A too rapid degradation in a closed container would result in that the required sterilizing dosage [f(time, concentration)] becomes too low.
Experiments were thus performed with PET bottles for the purpose of ascertaining whether the ozone formed in a closed container together with the ozone generating electron irradiation would be able to produce an effective sterilization by permitting the ozone to have a certain period of time to act.